The present invention relates to a scanning opthalmoscope for scanning human retinas and a method of scanning the retina of an eye.
An ultra-wide field scanning opthalmoscope for scanning human retinas is described in the Applicant's European Patent No. 0730428.
The system comprises:
                1. a slow scan element;        2. a fast scan element;        3. an ellipsoidal main mirror; and        4. a scan compensator.        
The slow scan element provides a scanning motion of an incident laser beam, in a first direction. The fast scan element provides a scanning motion of the incident laser beam, in a second direction, orthogonal to the first direction.
The fast scan element is positioned at the first focus point of the ellipsoidal main mirror and the subject's pupil is positioned at the second focus point of the ellipsoidal main mirror. Therefore, light emanating from the first focus point of the mirror is reflected through the second focus point of the mirror, and therefore, through the subject's pupil.
As a result of the apparent or virtual point source, the system enables ultra-wide retinal images of the retina to be obtained. The system enables taking 120 degree external scans through 2 mm undilated pupils.
However, in order to allow transmission of the laser beam, it was found necessary to make modifications to the system described above. The modifications provided a lateral spacing between each of the above-referenced components, and applied a “tilt” to the input laser beam.
This modified system was found to work well, transferring light efficiently over the substantial scan angles. However, the consequence of the tilted input beam was that the scan on the retina had a “shear” component which varies as a function of field position. While this shear distortion does not affect the ability to diagnose pathology, it does impact initial perceptions and the ability to measure consistent dimensions within the retinal images.
It is an object of the present invention to provide an improved scanning opthalmoscope which obviates or mitigates one or more of the disadvantages referred to above.